The invention relates to a process for continuously preparing heat-vulcanizable silicone compositions.
A heat-vulcanizable silicone composition comprises a high viscosity silicone polymer, an inorganic reinforcing filler and various additives that aid processing or impart desired final properties to the composition. A vulcanizing agent can be added and the composition heat-cured to fabricate silicone rubber moldings such as gaskets, medical tubing and computer keypads.
Typically, the heat-vulcanizable silicone composition is produced by kneading a high-viscosity polydiorganosiloxane, the inorganic filler and additives by means of a batch kneading machine such as a high intensity Banbury mixer or a low intensity double arm dough mixer. In this process, polydiorganosiloxane, inorganic filler, treating agents and additives are batch mixed until desired properties are obtained. In Kasahara et al., U.S. Pat. No. 5,198,171, a preconcentrate of polydiorganosiloxane, inorganic filler and treating agents is formed by a high speed mechanical shearing mixer. The resulting premix is further compounded in a same-direction double screw extruder. The premix is formed in a first step wherein a diorganopolysiloxane having a viscosity at 25xc2x0 C. of 1xc3x97105cP or more, an inorganic filler and a treating agent are mixed in a high speed mechanical shearing machine to provide a flowable particulate mixture in which each ingredient is present in a substantially uniform, finely dispersed state. The flowable particulate mixture is then fed at a constant feed rate into a kneading and extruding machine that has two screws rotating in the same direction.
A batch process requires long mixing times and large amounts of energy. Non-homogeneous shear and extensional stress across a commercial sized batch can result in non-uniform size distribution of filler that results in variations in properties. Batches processed at different times may be characterized by different physical properties. The batch process is labor, energy and capital intensive and produces materials of only marginal consistency.
In Hamada et al., U.S. Pat. No. 5,409,978, a preconcentrate of polydiorganosiloxane, inorganic filler and treating agents is formed at a temperature in the range of about 200xc2x0 C. to 300xc2x0 C. in a co-rotating continuous double screw extruder. The preconcentrate is then compounded and heat treated at 150xc2x0 C. to 300xc2x0 C. in a counter-rotating, double screw extruder. However, a process that requires two extruders is expensive and requires significant processing area.
However with these processes, throughput is limited. There is a need for a process that provides improved throughput and which can be adapted as a low cost process that can efficiently utilize a single extruder to continuously and consistently produce a full range of both low viscosity and high viscosity silicone elastomers from filler, additive and polymer.
The invention provides a process that compounds high levels of filler, processing fluid and silicone polymer into homogeneous filled heat-vulcanizable silicone compositions with requisite reinforcing properties and levels of volatiles. The process comprises continuously feeding filler and silicone polymer to a high speed mixing stage to form a free-flowing particulate concentrate. The concentrate is continuously discharged from the mixing stage to a compounding apparatus for further processing.
In another aspect, the invention relates to a process of forming a premix of filler and silicone polymer. In the process, a filler is mixed with a silicone polymer in a continuous annular layer mixer and a filled silicone polymer premix is discharged from the mixer.
In another aspect, the invention relates to a process of compounding a filled heat-vulcanizable silicone composition wherein a filler is mixed with a silicone polymer in a high speed continuous mixer at an element tip speed of between about 3 m/s and about 100 m/s to form a premix. The premix is then discharged to a next processing apparatus.
In another aspect, the invention relates to a continuous annular layer mixer having a sequence of sections comprising at least a first section comprising a forward pitched mixing element, a second section comprising a neutral or forward pitched cutting element and a third section comprising a rearward pitched mixing element.
In yet still another aspect, the invention relates to a compounding apparatus, comprising a first stage continuous annular layer mixer and at least one subsequent stage comprising an extruder connected to the first stage to permit continuous discharge of processed material from the first stage to the second stage.